Leap Year Checker

What makes a year a leap year?

A leap year has 366 days instead of 365 — one extra day inserted as February 29th. This keeps our calendar aligned with Earth's actual orbit around the Sun, which takes approximately 365.2422 days.

The Gregorian leap-year rule

A year is a leap year if:

1. It is divisible by 4,
2. unless it is divisible by 100,
3. unless it is divisible by 400.

So:

• 2024 — leap year (divisible by 4).
• 1900 — not a leap year (divisible by 100 but not 400).
• 2000 — leap year (divisible by 400).
• 2100 — not a leap year.
• 2400 — leap year.

Why the extra "except every 100, except every 400"?

Simply adding a day every 4 years overcorrects — it gives an average year of 365.25 days, but the real tropical year is slightly less. Skipping the leap day in century years (except every 400 years) brings the average down to 365.2425 days — accurate to about one day every 3,300 years.

Recent and upcoming leap years

2020, 2024, 2028, 2032, 2036, 2040, 2044, 2048, 2052 — and onward every four years, with 2100 skipped but 2400 kept.

Why we leap in February

In the original Roman calendar, February was the last month of the year, and leap days were inserted at the end of the calendar — which happened to be in February. The position stuck even after New Year moved to January.

Why we need leap years at all

Earth takes approximately 365.2422 days to complete one orbit around the sun — not a whole number. If our calendar had exactly 365 days every year, then over time the seasons would drift. After about 100 years, the error would accumulate to roughly 24 days — enough that "winter" would start showing up in early December instead of late December, and eventually our summer months would be cold. Leap years keep the calendar aligned with the actual solar year.

The Gregorian leap-year rules in full

A year is a leap year if:

1. It is divisible by 4, AND
2. It is NOT divisible by 100, OR
3. It IS divisible by 400.

Worked examples:

• 2024 — divisible by 4, not divisible by 100. Leap year.
• 2100 — divisible by 4, divisible by 100, not divisible by 400. Not a leap year.
• 2000 — divisible by 4, divisible by 100, divisible by 400. Leap year.
• 1900 — divisible by 4, divisible by 100, not divisible by 400. Not a leap year.
• 2400 — divisible by 400. Leap year.

The 400-year cycle

Over any 400-year stretch of the Gregorian calendar, there are exactly 97 leap years and 303 common years, giving a total of 146,097 days. Divide by 400 and you get a mean year of 365.2425 days — just 27 seconds longer than the actual tropical year. This means the Gregorian calendar drifts by one day every ~3,300 years.

Leap years in other calendars

• Julian calendar — a leap year every 4 years without exception, giving a mean year of 365.25 days. This is why the Julian calendar has drifted 13 days ahead of the Gregorian in the past 2,000 years.
• Persian calendar — uses a 33-year cycle with 8 leap years. Even more accurate than the Gregorian.
• Hebrew calendar — 7 leap years in every 19-year cycle, adding an extra month (not an extra day).
• Islamic calendar (tabular) — 11 leap years in every 30-year cycle, adding one day to Dhu al-Hijjah.
• Ethiopian / Coptic — every 4th year is a leap year, matching the Julian rule.

The Pope's correction of 1582

When Pope Gregory XIII introduced the Gregorian calendar in October 1582, he did two things: refined the leap year rule (the new "skip every 100 except every 400" exception), and cut 10 days out of October to realign the calendar with the seasons. Thursday 4 October 1582 was followed directly by Friday 15 October 1582. No days existed between them. Contracts, birthdays, saints' days — all had to be recalculated. Some countries took over 300 years to adopt the change; Britain and its colonies switched only in 1752, losing 11 days to sync up.